Ester production using noble metalbromine compound catalysts



May 24, 1966 W. D. SCHAEFFER ESTER PRODUCTION USING NOBLE METAL-BROMINE COMPOUND CATALYSTS Filed July 23@ 1962 aff-wv BY/ E United States Patent 3,253,020 ESTER PRODUCTION USENG NOBLE METAL- BRGMNE CMPUUND CATALYSTS William D. Schaeffer, Pomona, Calif., assignor to Union Oil Company of California, Los Angeles, Calif., a corporation of California Filed July 23, 1962, Ser. No. 211,820 Claims. (Cl. 2613-497) This invention relates to a process for the preparation of unsaturated esters and, in particular, relates to the oxidation of ethylene lto vinyl acetate.

Vinyl esters are commonly obtained by the vinylation of carboxylic acids with acetylene. While this reaction is relatively simple and provides high yields of vinyl esters, it is based on expensive raw materials-acetylene and acetic acid. The cost of vinyl esters could be significantly reduced by a process based on relatively inexpensive ethylene. In addition, a process is desired for commercial production of unsaturated esters from other hydrocarbon olens, e.g., propylene, butene-l, etc., to yield commercial quantities of esters such as propenyl acetate, propenyl butyrate, butenyl acetate, pentenyl propionate, etc.

It is an object of this invention to provide for the oxidation of hydrocarbon olens to unsaturated esters.

It is a specific object of this invention to provide for the oxidation of ethylene to vinyl acetate.

Other and related objects will be apparent from the following discussion.

l have found that hydrocarbon olefins can be oxidized to unsaturated esters by contacting the olefin with oxygen and a carboxylic acid in the presence of a Group Vlll noble metal and bromine. In a specific embodiment, oxygen and ethylene are continuously admixed with acetic acid containing between about 0.05 and about 2.0 Weight percent bromide as a soluble bromide salt, dissolved bromine or hydrobromic acid and also containing between about 0.()5 and about 2.0 weight percent of a Group VIII noble metal, preferably, palladium as a dissolved salt, eg., palladium bromide and/ or suspended in metallic form.

Referring now to the olefins which can be oxidized to an unsaturated ester in accordance with my invention, any of the following can be used:

wherein R1 and R2 are hydrogen, aryl, e.g., phenyl, naphthyl, tolyl, etc., alkaryl, e.g., benzyl, p-cuminyl, etc.; alkyl, e.g., methyl, lauryl, isopropyl, etc. Examples of suitable olens are: ethylene, propylene, butene, pentene, hexene, heptene, cyclohexene, indene, styrene, allyl benzene, etc. Ethylene is preferred to obtain vinyl esters; however, isopropenyl esters canrbe obtained from propylene and, in general, 1-olens yield products esteriiied at the 2-carbon. In general, any olefin having one or more hydrogens on each of the carbons bearing the double bond can be esteried; alkenes having 2 to about 5 carbons are preferred raw materials.

The ester group which is reacted with the olen can be widely varied and is determined by the anions available in the solution. Preferably, a carboxylic acid is employed as the liquid reaction medium and this acid serves as the esterifying reactant. lf desired, however, salts of various organic acids can be added to the reaction liquid to serve as a source of esterifying anion. eral, alkyl, aryl, alkaryl monoand di-carboxylic acids can be used which have between 1 and about 15 carbons; the lower (C1 to C5) aliphatic acids are preferred and of these, acetic is most preferred because of the established commercial value of vinyl acetate. Examples of suitable acids are: formic, acetic, propionic, butyric, iso- In genice butyric, valeric, isovaleric, caproic, isocaproic, caprylic, isocaprylic, benzoic, terephthalic, oxalic, succinic, glutaric, adipic, pimelic, suberic, azelic, lauric, nitrobenzoic, toluic, naphthoic, etc., or mixtures thereof. Various combina- -tions of the aforementioned can be used, e.g., acetic acid can serve as the reaction medium and various salts; preferably alkali metal salts; of any of the aforementioned acids can be added to furnish an esterifyiug anion, eg., sodium butyrate, potassium terephthalate, lithium benzoate, sodium propionate, cesium valerate, etc. As previously mentioned, however, a lower aliphatic acid is used as the preferred reaction medium and source of esterifying anion.

Exa-mples of various unsaturated esters which can be prepared by my 'process are: vinyl acetate, propenyl acetate, butenyl acetate, pentenyl acetate, phenylethenyl acetate, phenylpropenyl acetate, cyclohexenyl acetate, vinyl propionate, vinyl isobutyrate, propenyl valerate, propenyl benzoate, divinyl adipate, monopropenyl succinate, phenylethenyl benzoate, divinyl terephthalate, propenyl naphthalate, etc.

As previously mentioned, a Group VIII noble metal in combination with bromine is the catalyst for the oxidation. Of the noble metals, platinum, rhodium, ruthenium, osmium, iridium and palladium, the latter is preferred because of its relatively high catalytic activity. In general, the metal is employed in amounts between about 0.001 and 5.0 weight percent of the liquid reaction medium; preferably between about 0,05 and about 2.0 weight percent. The noble metal can be added to the liquid medium as finely divided metal, as a soluble salt or as a chelate.v Examples of suitable salts are the halides and acetates. Preferably the bromine salts, eg., palladium bromide is used. Examples of suitable chelates are palladium acetylacetonate, and complexes of the metal ion with such conventional chelating agents such as ethylene diamine tetraacetic acid, citric acid, etc.

The bromine is added to the organic liquid as a soluble bromine salt,' hydrogen bromide or elemental bromine.

Preferably, hydrogen bromine or a soluble bromine salt is employed. Examples of suitable salts are alkali metal, eg., sodium, potassium, lithium, cesium bromides; ammonium bromide or the aforementioned Group VIll noble metal bromides. vIn general, suicient of the aforementioned bromine containing materials should be added to provide a concentration of between about 0.05 and about 2.0 weight percent; preferably between about 0.1 and about 0.5 weight percent of bromine in the liquid reaction medium.

The reaction conditions are relatively mild, temperatures between about 50 and about 600 F. can be used; preferably between about and about 400 F. The reaction pressure is suicient to maintain the carboxylic acid in liquid phase at the selected reaction temperature, generally between about l and about 50 atmospheres being sufficient.

To inhibit polymerization of the unsaturated ester product and to minimize the yield of aldehyde or ketone by-product, it is desirable to employ short .reaction periods, generally from several seconds to about 30 minutes; preferably reaction periods of from about 5 to about 20 minutes are employed. Suitably the desired reaction period can be obtained by concurrently flowing a liquid stream of the bromide and palladium -catalyst in carboxylic acid through the reaction zone with a stream of olefin and oxygen.

The olefin-is supplied to the reactor in excess quantities so as to inhibit oxidation of the unsaturated ester product vto a more highly oxidized product, the excess unconverted gas being recycled to the reactor with fresh volumes of olefin.

. In general, maximum yields of ester product are obtained when `the amount of water in the reaction zone does not exceed about l weight percent of the liquid reactants; preferably the water content is maintained less than weight percent and, most preferably, less than 1 weight percent in the reaction zone. If desired, acetic anhydride or other suitable dehydrating agents can be added to the reaction medium to maintain the system anhydrous during the oxidation. In general, 5 to 90 percent, preferably from to 50 percent, of the liquid reaction medium can be acetic anhydride, the actual amount used being sufficient to combine with the water formed by the oxidation. In this manner, high conversions to the ester product can be achieved while inhibiting the formation of aldehyde or ketone by-products.

The oxygen containing gas such as air, oxygen or mixtures thereof is admixed with the olefin feed, preferably in suitable proportions to avoid expolsive mixtures. In general, the maximumoxygen concentration should be less than about 5 volume percent of the gas mixture; preferably between about l and 3 volume percent of the gas to the reactor.

The figure illustrates a continuous process based on my invention. A reactor 1 suitably lined with a corrosion resistant material, e.g., tantalum, titanium, Teflon or a ceramic, contains the liquid reaction medium which is supplied thereto through line 2. The liquid is supplied at a rate suflicient to obtain the desired residence time in the reactor.

The olefin and oxygen source are supplied to the reactor through lines 3 and 4, respectively, in the aforementioned proportions, the olefin being in excess to prevent oxidation of the ester product and the oxygen supplied in amounts insufficient to form explosive gas mixtures.

The ester product is removed with the reaction medium from the reactor at line 5 and is passed to a distillation tower 6 where the dissolved gases, ethylene, oxygen, nitrogen and the low boiling aldehyde or ketone byproduct are flashed from the liquid and removed overhead lthrough line 7. The aldehyde or ketone by-product is condensed in cooler 8 and passed to distillate drum 9 where the gases are separated and vented into gas line 10 via line 11. The condensate is withdrawn by line 12, pump 13 and used to reux the tower through line 14. A continuous stream of by-product aldehyde or ketone can be recovered as product through line 15 or sent to reactor 40, hereinafter described, through line 16.

The liquid residue from tower 6 contains ester product, Water, some polymerized ester, and organic solvent and catalyst. This liquid is sent to product distillation 4tower 20 through line 21 where the ester product is recovered. When the ester azetropes with water, the distillation is performed so as to remove the bulk of the water from the reaction medium in this step and provide a residue which can be directly recycled `to the reactor through line 22, pump 23 and linev 24 to line 2.

The ester-water azeotrope overhead from tower is passed to cooler 25 and to condensate drum 26. This condensate drum is baied to separate an aqueous phase which is removed by line 27 and pumped to a water stripper 28 with pump 29. A stripping gas, e.g., ethylene, is supplied through line 50 to remove traces of ester product from the waste water by azeotropic topping and the stripper overhead can be combined with tower 20 overhead through line 30 Ito line 21 upstream of cooler 25.

The ester product is removed from drum 26 by line 31, pump 32 to storage through line 33. A suicient amount of ester is used to reflux tower 20 through line 34 and this reflux rate is controlled in response to the water content of the liquid residue from tower 6 to remove all the water in azeotropic distillation zone 20.

To remove the polymer by-product, and other high boiling by-products, a portion of the liquid residue from tower 20 is sent to reactor 40 by lines 35 and 16. In reactor 40 the liquid is contacted with an oxygen containing gas -to oxidize the by-p-roducts to a carboxylic acid. In general, temperatures between about 200 and about 500 F. are used and preferably the oxidation is conducted in the presence of a suitable oxidation catalyst such as a heavy metal having an atomic number of 23 to 28 inclusive. Examples of these are manganese, cobalt, nickel, chromium, vanadium, molybdenum, tungsten, tin, cerium and mixtures thereof. The metal is added to reactor 40 in catalytic amounts, between about 0.01 and about 5 weight percent, as a soluble salt such as the acetate, chloride, bromide, etc., as iinely divided metal or as a chelate with acetyl acetone, ethylene diamine tetraacetic acid, etc. It is also preferred to have between about 0.01 and 2.0 weight percent of a bromine containing substance in this oxidation zone to promote the oxidation; generally no bromine need be added since sufficient bromine is in the liquid residue from tower 20; however, if necessary any of the aforementioned bromine compounds can be added to reactor 40. The aldehyde or ketone by-product such as acetaldehyde, acetone, etc., which is separated in line 14 can also be combined with the liquid residue of line 35 to convert all the by-product materials to acetic acid to replace all or a portion of the acetic acid consumed in reaction 1 in the esterication of ethylene Fresh volumes of the carboxylic acid are added through line 37 when necessary.

The oxidized liquid from reactor 40 is passed to distillation tower 51 through line 38 where the uncondensibles and water are removed overhead by line 39. Preferably, azeotropic distillation is used by the addition of a suitable azeotroping Ihydrocarbon such as benzene to the distillation zone in suicient quantities to remove the water from the crude oxidate. The residue comprises the carboxylic acid (acetic) and catalyst which is recycled by line 41 and pump 42 to line 2. Fresh catalysts, i.e., noble metal and/or bromine is added at 43 when necessary. When a heavy metal oxidation catalyst is used in reactor 40, it of course is also present in the residue recycled to reactor 1. I have found that the ester manufacture is not deterred by the presence of this heavy metal; indeed there is reason to believe that some of the metals, notably cobalt, tend to promote the oxidation in reactor 1 -in that they serve to oxidize palladium metal to palladous ion in acetic acid.

The overhead in line 39 is cooled at 44 and passed to drum 45 where the non-condensibles are separated by line 46 and combined with the gases in line 10. An aqueous layer is separated from drum 45 by line 47 and discarded, although suitable acetic acid recovery treatments can be used to recover the traces of acetic acid in the water.

The following examples will illustrate the yields of unsaturated ester products obtainable by my process:

Example l A solution containing 530 grams acetic acid, 6 grams lithium acetate dihydrate, 2 grams of hydrobromic acid (48% aqueous solution) and 70 ml. of benzene was distilled through a 20-plate Oldershaw column to dry the mixture. The distillation was continued until the overhead temperature reached the boiling point of pure acetic acid. The residue weighed 513 grams.

The residue together with 1.0 gram of metallic palladium was charged to a Teflon-lined autoclave and ethylene added to 500 p.s.i.g. The mixture was heated to 300 F. then nitrogen was added to 900 p.s.i.g. Oxygen was then added to 930 p.s.i.g. which resulted in an immediate reaction as evidenced by a temperature increase and a pressure drop on the system. Additional quantities of oxygen were added as required to maintain the pressure near 900 p.s.i.g. for the 20 minute reaction period. The reaction was terminated by cooling the mixture to Material Grams Mol Percent Acetaldehyde 2.4 12. 7 Vinyl acetate 28.1 75.7 Ethylidene diacetate. 4. 6. 2 Polyviuyl acetate 2. U 5. 3

Example 2 When Example l was repeated with an equivalent weight 4rhodium in lieu of the palladium, vinyl acetate was also produced. Similarly, comparable yields of vinyl acetate can be obtained by addition of l gram of platinum in lieu of the palladium of the example.

Example 3 Example l was repeated, except no hydrogen bromide was added. After a thirty minute reaction period there was no increase in the weight of liquid and no products boiling below acetic acid were formed.

Example 4 Example l was repeated in the absence of any noble metal but with 4 grams of 4S percent strength hydrobromic acid and no ester products were formed.

Example 5 To a mixture of 455 grams acetic acid and 45 grams of water was added 1.5 grams palladium chloride and 3.0 grams of 48 percent strength hydrobromic acid. This solution was charged to the Teon lined autoclave and ethylene was added to a pressure of 500 p.s.i.g. The autoclave was heated to 300 F. and pressured to 900 p.s.i.g. with nitrogen. Oxygen was added in the manner described in Example l over a one-hour period. The weight increase was 40 grams with a 74.2 mol percent yield of acetaldehyde and a 25.4 mol percent yield of vinyl acetate. Repetition of the preceding with equal volumes of water and acetic acid failed to yield significant amounts of oxidized product.

Example 6 Material Grams Mol Percent Acetaldehyde 7. 2 8.0 Vinyl acetate 63.0 36. 0 E thylideue diacetate 50. 0 16. 8 Polyvinyl acetate 69. 0 39. 2

Example 7 The autoclave was charged with l gram of palladium chloride, l gram of 48 percent strength -hydrobromic acid, 6 grams of lithium acetate, 400 grams of acetic anhydride and 100 grams of acetic acid. The autoclave was then pressured to 500 p.s.i.-g. with ethylene, heated to 300 F., pressured to 900 p.s.i.g. with nitrogen and oxygen thereafter was introduced incrementally over a one-hour period. The weight increase was 4l grams and the yield of products was:

Materiai Grams M01 Percent Acetaldehyde- 2. 2 7. 2 Vinyl acetate 42. 4 70. 6 Etlxylideue diacetate 9. 0 8.9 Polyvinyl acetate 8. 0 13. 3

Example 8 Example 7 was repeated with 500 grams of acetic acid in lieu of the acetic anhydride-acetic acid mixture previously employed. A 40-gram weight increase was observed and the yield of products was:

Material Grams Mol Percent Acetaldehyde 9. 6 33. 9 Vinyl acetate 25.5 46. 3 Ethylidene diacetate 5. 0 5. 3 Polyvinyl acetate 8. 0 14. 5

The preceding examples are intended only to illustrate my invention and are not to be construed as unduly limiting thereof. My invention is intended to be defined by the method steps and equivalents thereof set forth in the following claims.

I claim:

1. A continuous process for the preparation of an unsaturated ester of a carboxylic acid that comprises contacting a hydrocarbon olefin having from 2 to about 5 carbons with a reaction medium consisting essentially of a carboxylic acid having from l to about l5 carbons, less than l0 `weight percent water, `from 0.001 to 5.0 weight percent of a Group VIII noble metal, and from 0.05 to 2.0 weight percent of a soluble bromide selected from the class `consisting of alkali metal, ammonium and hydrogen bromide at a temperature from about 50 to 600 F. and sufficient pressure to maintain said reaction medium in liquid phase While concurrently introducing oxygen into contact with said reaction medium.

2. The method of claim 1 wherein the reaction medium also comprises acetic anhydride.

3. The oxidation of ethylene to acetaldehyde and the vinyl ester of a carboxylic Aacid which comprises contacting said ethylene with oxygen in the presence of a liquid reaction medium consisting of said carboxylic acid and less than l0 weight percent Water which contains amounts of from about 0.001 to 5 .0 weight percent palladium and from about 0.05 to 2.0 weight percent of a soluble bromide selected from the class consisting of alkali metal, ammonium and hydrogen bromides at a temperature between about 50 and about 600 F. and a pressure sufficient to maintain said liquid phase.

4. The oxidation of claim 3 wherein said reaction medium also contains about l `weight percent of an alkali metal salt of said carboxylic acid.

5. A continuous preparation of vinyl acetate that comprises contacting ethylene with a reaction medium consisting essentially of substantially anhydrous acetic acid, about l weight percent of an alkali metal acetate and amounts of from about 0.001 to 5.0 weight percent palladium and from about 0.05 to 2.0 weight percent of hydrogen bromide at a temperature between about and about 400 F. and superatmospheric pressures to thereby produce said vinyl acetate while simultaneously maintaining the activity of said reaction medium by introducing oxygen into contact with said reaction medium concurrently with the introduction of said ethylene.

(References on following page) 7 8 References Cited by the Examiner OTHER REFERENCES UNITED STATES PATENTS Moiseev et al.: Proceedings of the Academy of Sciences,

U.S.S.R., v01. 133, pages 801 to 804, July 1960. 2519754 8/1950 Gresham et 3 1 26o-497 Moiseev: Russian Academy of Sciences, Physical Chem- 3,057,915 10/1962 Riemenschneider et a1.260-604 5 ism-y, 1960 p' 116 l AngeW. Chem., V01. S Smidt: Chemistry and Industry, Ian. 13, 1962, p. 54.

608,610 3/1962 Belgium. 618,144 4/1961 Canada- LORRAINE A. WEINBERGER, Primary Examiner.

137,511 4/ 1960 U.S.S.R. 10 LEON ZITVER, Examiner. 

1. A CONTINUOUS PROCESS FOR THE PREPARATION OF AN UNSATURATED ESTER OF A CARBOXYLIC ACID THAT COMPRISES CONTACTING A HYDROCARBON OLEFIN HAVING FROM 2 TO ABOUT 5 CARBONS WITH A REACTION MEDIUM CONSISTING ESSENTIALLY OF A CARBOXYLIC ACID HAVING FROM 1 TO ABOUT 15 CARBONX, LESS THAN 10 WEIGHT PERCENT WATER, FROM 0.001 TO 5.0 WEIGHT PERCENT OF A GROUP VIII NOBLE METAL, AND FROM 0.05 TO 2.0 WEIGHT PERCENT OF A SOLUBLE BROMIDE SELECTED FROM THE CLASS CONSISTING OF ALKALI METAL, AMMONIUM AND HYDROGEN BROMIDE AT A TEMPERATURE FROM ABOUT 50* TO 600*F. AND SUFFICIENT PRESSURE TO MAINTAIN SAID REACTION MEDIUM IN LIQUID PHASE WHILE CONCURRENTLY INTRODUCING OXYGEN INTO CONTACT WITH SAID REACTION MEDIUM. 